Since automobile tires are manufactured using rubber compositions made from natural rubber and/or synthetic diene rubbers, degradation of such tires is accelerated at high ozone concentrations or under ultraviolet rays, which may cause cracking, for example, tread groove cracking (TGC) at the base of the main grooves of the tread. In order to suppress crack formation and growth in the presence of ozone, for example, additives such as antioxidants, e.g., N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) or poly(2,2,4-trimethyl-1,2-)dihydroquinoline (TMDQ), or petroleum wax are used in rubber compositions.
The antioxidants and petroleum wax in rubber vulcanizates migrate (bloom) to the rubber surface of, for example, tires during vulcanization or service, thereby serving to protect the rubbers from ozone. However, if the antioxidants and petroleum wax are excessively added beyond the solubility of the rubber, they excessively bloom during the initial service period, causing white discoloration. Moreover, the antioxidants oxidized by ozone cause brown discoloration, and similar excessive blooming of them intensifies brown discoloration. Thus, it has been difficult to suppress crack formation while preventing discoloration.
With the recent increased demand for lighter weight tires, the sub-tread thickness at the base of the tread main grooves is being reduced to reduce tire weight. Unfortunately, however, the lighter tires are more likely to suffer from TGC during storage at tire dealers, stores, or the like. TGC is more likely to occur particularly when tires are stored in a state where the rubber at the base of the tread main grooves is strained such as, for example, when tires (10 to 15 tires) are stacked flat (horizontally) on each other or tires are forced into a narrow upright storage rack space so that they are deformed; or when they are stored in a high ozone environment such as, for example, in a dry season, around an ozone deodorizer or electric sparks produced by a rotating motor, or the like. Presumably, as the sub-tread thickness at the base of the tread main grooves is reduced, the rubber at the base of the tread main grooves is more likely to be strained and therefore TGC is more likely to occur during storage.
Patent Literature 1 describes that the addition of a polyoxyethylene ether nonionic surfactant prevents deterioration of the appearance of tires. This technique still leaves room for improvement in terms of preventing discoloration while exhibiting good TGC resistance even after the tire having a reduced sub-tread thickness at the base of the tread main groove is stored.